The present invention relates to improvements in transformers for use in electric circuits and to a method of manufacturing an improved transformer assembly.
While the principal features of the transformer assembly and the method according to the present invention may be incorporated into transformers for use in various types of electric devices and utilized for the manufacture of such transformers, the transformer assembly provided by the present invention will prove useful especially as a flyback transformer to be incorporated into the horizontal deflection circuit of a cathode-ray tube of a television receiver.
A flyback or horizontal deflection output transformer of a television receiver is used to produce a horizontal deflection current during the scanning period and to develop a high anode voltage of the cathode-ray tube from the pulse voltage induced in the primary circuit of the transformer during the flyback or retrace period.
To cope with the increasing requirement for the safety of operation and the reduction of the cost and size of a television receiver, there is a trend to have the flyback transformer installed on a printed circuit board together with various other circuit elements of the television receiver. Research and development efforts are thus being made in the art in an attempt to provide a flyback transformer which can be readily and securely mounted on a printed circuit board without taking up much space and affecting the performance characteristics of the electric elements located in the neighbourhood of the transformer.
While varying in detailed construction depending upon the types, sizes and desired performance characteristics, a flyback transformer generally has a magnetic circuit taking the form of a single ring encircled by two or more groups of primary and secondary windings which are distributed around the periphery of the ring, or ferromagnetic core. The primary and secondary windings are embedded in insulating envelopes of dielectric material such as a silicon resin filling the interstices between the adjacent windings and the layers of the windings and encapsulating the entire bodies of the windings. The primary and secondary windings or the secondary winding and an adjacent portion of the ferromagnetic core are thus spaced apart from each other through a dielectric layer of air that tends to produce a corona discharge. To prevent the production of a corona discharge between the electrodes thus constituted by the primary and secondary windings or the secondary winding and the adjacent portion of the core through such an air layer, it is important that the core be formed with a window which is large enough to accommodate a sufficiently large sized air layer or to permit the windings to be encased in sufficiently thick insulating envelopes. This results in increases in the size and weight of the core structure and accordingly in those of the transformer as a whole and provides difficulties in securely mounting the transformer on a printed circuit board at a low cost and by simple steps.
The core structure of a flyback transformer is usually formed with air gaps to prevent magnetic saturation of the core structure due to the flow of a current in the primary winding. Leakage flux is produced from these air gaps of the core structure and from exposed areas, if any, of the primary and secondary windings and tends to cause inductive interference with other electric circuits and circuit elements located in the neighbourhood of the transformer. To protect the circuits and circuit elements from such a disturbance, it is important that the circuits and circuit elements be located at sufficient spacings from the transformer. If this is to be performed in a printed circuit board, an extra space is required of the printed circuit board and makes it necessary to use a large-sized circuit board. If, furthermore, the flyback transformer is located in proximity to the cathode-ray tube, then the leakage flux of the transformer causes the electron beams to deviate from the paths determined by the deflection coils and spoils the purity of the image reproduced on the faceplate especially in a colour television receiver. The flyback transformer should therefore be located at a sufficient spacing from the cathode-ray tube. If it is undesired or impossible to obtain such a spacing in a television receiver set by reason of the specific configuration of the printed circuit board to be in use or the desired configuration of the cabinet, a shield element must be provided between the flyback transformer and the cathode-ray tube so as to magnetically isolate the cathode-ray tube from the leakage flux of the transformer. To prevent production of a corona discharge between the shield element and the secondary winding used as a high-voltage circuit, the secondary winding of the flyback transformer should be sufficiently spaced apart from the shield element, requiring the use of a large-sized receiver cabinet to provide such an extra space. The video image to be produced on the faceplate will also be degraded by the charges which are stored on the core structure and discharged onto the circuit elements on the printed circuit board and onto some metal elements forming part of the flyback transformer such as bolts and nuts securing the terminal plate to the core structure.
The flyback impulses induced in the windings of a flyback transformer produce mechanical vibrations in the core structure on which the secondary winding is carried. The vibrations are transferred to the printed circuit board supporting the transformer and through the circuit board to the various circuit elements on the board and produce unpleasant noises between each of the circuit elements and the printed circuit board and between the circuit board and the chassis on which the circuit board is mounted.
In addition to these drawbacks, problems have been encountered during assemblage of prior art flyback transformers, especially in impregnating the primary and secondary windings with a dielectric material in a mould, fastening the terminal plate and other complementary component parts to the core structure by bolts and nuts, and adjusting the sizes of the air gaps to be formed in the core structure.